Having just finished successfully installing Linux on a Toshiba Tecra 550CDT, using the almightly Slackware 8.1 distro, I have some information to share with any of you who want to get it going on such.

First of all, the floppy is not needed if you're using a Slackware install CD, or a bootable CD from another distro.

BIOS SETUP: To get into setup on a 550CDT, or most other Toshiba laptops, hold the ESC key while turning on or rebooting the system. You should be rewarded with a rather loud and obnoxious beep and a "Check system. Then press F1 Key." message. Pressing F1 will enter the setup utility. The first thing you want to do here is change the boot order so that your CDROM will be checked before the first hard disk drive.
Other things you may want to set:
1. CardBus/PCMCIA mode: I've found everything works ok with it set to "CardBus/16 bit". If you have trouble with pcmcia or cardbus cards, try setting Auto-selected. Note: If you're also running Windows on this system, changing this setting will make Windows want to reinstall the Cardbus controller drivers... so have your Winhoze disc handy.
2. LCD Display Stretch: Unless you like that little postage-stamp size screen mode, enable this!
Now, for the second page of setup... page down gets you to this.
I think the only thing that really needs to be set manually here is the sound parameters - defaults should be fine, but write them down! You'll need these values later. On mine, after it was used with Windows, all of the configuration options were set to "Not Used" so Windows could handle them via Plug 'N' Pray. Anyway... with all this set, you can get around to installing.

Insert the install CD, exit setup with the End key, and you should be booting up into your distro's installer.

Partitioning: The swap partiton should be 2.5 times the amount of RAM in the machine, generally. On mine, with 64 megs, this is 192 megs, give or take a bit. Note that placing the swap partition closer to the beginning of the drive will increase performance, due to the drive using zoned-bit recording.

Once you get everything installed and have rebooted into your new system, there will be a couple things missing. The first of these is the modem - I have yet to figure out just how to get this working, so I'm just using a USRoboticsPC Card V.90 modem. The modem is rather odd in this machine; you can manually set it to a certain COM port in the BIOS setup, however, it is a winmodem and won't work without proper support.
The second thing you won't have just yet is sound; here's how to get that working. Edit /etc/rc.d/rc.modules and add these lines to the sound section:
(this example assumes an io address of 0x530, set in the BIOS; see /usr/src/linux/Documentation/sound/CS4232 for more info)
modprobe sound
insmod ad1848
insmod uart401
insmod cs4232 io=0x534 irq=5 dma=1 dma2=0
insmod opl3 io=0x388

Now, if you want to configure XFree86 (you probably do), run xf86config. The pointer stick is a standard PS/2 mouse... enable the ChordMiddle option, since there's no third button. Videocard is an S3 VIRGE/MX, with 4096K video RAM. The screen works for me as "Super VGA 1024x768 @ 56 hz". I have only the 1024x768 modes enabled, since that's the display's native resolution. I think that about covers the quirks of the X configuration.

After getting everything else set up, I went on to compile a custom kernel (the one Slackware installs by default is an i386 kernel, so it may not be as fast or small as possible). The CPU in the 550CDT is a Pentium-MMX class. I don't know if it benefits from MTRR support, but enabling APM and Toshiba Laptop Support definitely sound like good ideas.

While on the subject of power management... note that your keyboard has a little faucet key (Fn + F2). This cycles through the machine's power modes in this order: High, Low, User Define. You can set the specific parameters for User Define in the BIOS setup... I have mine set with the LCD panel on semi-bright, CPU sleep on, cooling mode performance, cpu speed low, and can get over 3 hours battery life this way.

If you have any questions about installing on a 550CDT, feel free to /msg me, and I'll either answer your question or forward it to my technical support department (staffed entirely by guppies).